DH came home very upset

I agree with you Sydney. There does come a time when they realize in their heart and head that they are only one person who can only do so much. They will do what they can for whoever they can but can't save the world. I think it bothers them at first but over time they start shutting some of it out. There were a number of times when I had to tell me DH "NO. Stop. I don't want to hear that". He has started monitoring what he tells me about certain specific things that he knows I don't want to hear. Like them we start to harden (or toughen) up too. It may sound wrong. It may sound bad. But they and we need that protection that we get from that wall being slowly built up. We'd all go crazy without it.

My SO is a paramedic and has been for several years. Its common for people to ask what is the worst call you've ever gone out on. For a long time he always went back to his days as a EMT-B and his first few weeks on the job. He went out on a 2yr old full arrest and holding the child in his arms he passed away. There was nothing more they could have done but it still gave him a lot of heart ache.
In the medical field, he sees a lot of things he would like to forget but children are his soft spot. Suicides, murders, rapes etc are just another day at work and he tells me about them to get it off his chest and he never speaks of it again. He has become numb to the things he sees.
On the other hand, runs on children are still painfully hard for him. Several months back he went to a home for a 4yr old having a seizure. Upon entering the house, he found her mother boyfriend doing cpr trying to save her life and an upset mother. He takes over cpr and they rush the little girl to the truck. Upon inspection of her body he feels ribs that had been fractured and healed incorrectly, bruises and burns all over her body. They worked for 30mins on the way to the hospital and the hospital continued to try and save her for several more mins but all their efforts were useless. The little girl died in the ER that night. Post mortem exam and xray showed she had suffered abuse by the hands of her caregivers for sometime.
He didn't really talk to me about this run. He kept it bottled up until he was ready to talk. When he heard her mother and boyfriend were in custody is when he opened up and broke down. He is reminded of that little girl every time they go to the housing development where she lived and hears her name. He has already been called to testify in the trial this summer.
I guess what I am getting at with this story is that, your dh is going to see things that he wants to forget and over time he will. Its cold to say he will become numb to it but I think its the best way for them to cope. Be there for him when he needs to talk and also know when to leave him alone and allow him to process it on his own.
Being with someone in fire, EMS and police field is not easy but as they grow in their careers and learn from their experiences, so do we as their SO'S.